Monoclonal antibodies, their conjugates and derivatives are hugely commercially important as therapeutic and diagnostic agents. Non-human antibodies elicit a strong immune response in patients, usually following a single low dose injection (Schroff, 1985 Cancer Res 45:879-85, Shawler. J Immunol 1985 135:1530-5; Dillman, Cancer Biother 1994 9:17-28). Accordingly, several methods for reducing the immunogenicity of murine and other rodent antibodies as well as technologies to make fully human antibodies using e.g. transgenic mice or phage display were developed. Chimeric antibodies were engineered, which combine rodent variable regions with human constant regions (e.g., Boulianne Nature 1984 312:643-6) reduced immunogenicity problems considerably (e.g., LoBuglio, Proc Natl Acad Sci 1989 86:4220-4; Clark, Immunol Today 2000 21:397-402). Humanized antibodies were also engineered, in which the rodent sequence of the variable region itself is engineered to be as close to a human sequence as possible while preserving at least the original CDRs, or where the CDRs from the rodent antibody were grafted into framework of a human antibody (e.g., Riechmann, Nature 1988 332:323-7; U.S. Pat. No. 5,693,761). Rabbit polyclonal antibodies are widely used for biological assays such as ELISAs or Western blots. Polyclonal rabbit antibodies are oftentimes favored over polyclonal rodent antibodies because of their usually much higher affinity. Furthermore, rabbit oftentimes are able to elicit good antibody responses to antigens that are poorly immunogenic in mice and/or which give not rise to good binders when used in phage display. Due to these well-known advantages of rabbit antibodies, they would be ideal to be used in the discovery and development of therapeutic antibodies. The reason that this is not commonly done is mainly due to technical challenges in the generation of monoclonal rabbit antibodies. Since myeloma-like tumors are unknown in rabbits, the conventional hybridoma technology to generate monoclonal antibodies is not applicable to rabbit antibodies. Pioneering work in providing fusion cell line partners for rabbit antibody-expressing cells has been done by Knight and colleagues (Spieker-Polet et al., PNAS 1995, 92:9348-52) and an improved fusion partner cell line has been described by Pytela et al. in 2005 (see e.g. U.S. Pat. No. 7,429,487). This technology, however, is not widely spread since the corresponding know-how is basically controlled by a single research group. Alternative methods for the generation of monoclonal antibodies that involve the cloning of antibodies from selected antibody-expressing cells via RT-PCR are described in the literature, but have never been successfully reported for rabbit antibodies.
Rabbit antibodies, like mouse antibodies are expected to elicit strong immune responses if used for human therapy, thus, rabbit antibodies need to be humanized before they can be used clinically. However, the methods that are used to make humanized rodent antibodies cannot easily be extrapolated for rabbit antibodies due to structural differences between rabbit and mouse and, respectively, between rabbit and human antibodies. For example, the light chain CDR3 (CDRL3) is often much longer than previously known CDRL3s from human or mouse antibodies.
There are few rabbit antibody humanization approaches described in the prior art, which are, however, no classical grafting approach in which the CDRs of a non-human donor are transplanted on a human acceptor antibody. WO 04/016740 describes a so-called “resurfacing” strategy. The goal of a “resurfacing” strategy is to remodel the solvent-accessible residues of the non-human framework such that they become more human-like. Similar humanization techniques for rabbit antibodies as described in WO 04/016740 are known in the art. Both WO08/144757 and WO05/016950 disclose methods for humanizing a rabbit monoclonal antibody which involve the comparison of amino acid sequences of a parent rabbit antibody to the amino acid sequences of a similar human antibody. Subsequently, the amino acid sequence of the parent rabbit antibody is altered such that its framework regions are more similar in sequence to the equivalent framework regions of the similar human antibody. In order to gain good binding capacities, laborious development efforts need to be made for each immunobinder individually.
A potential problem of the above-described approaches is that not a human framework is used, but the rabbit framework is engineered such that it looks more human-like. Such approach carries the risk that amino acid stretches that are buried in the core of the protein still might comprise immunogenic T cell epitopes.
To date, the applicants have not identified a rabbit antibody, which was humanized by applying state-of-the-art grafting approaches. This might be explained by fact that rabbit CDRs may be quite different from human or rodent CDRs. As known in the art, many rabbit VH chains have extra paired cysteines relative to the murine and human counterparts. In addition to the conserved disulfide bridge formed between cys22 and cys92, there is also a cys21-cys79 bridge as well as an interCDR S-S bridge formed between the last residue of CDRH1 and the first residue of CDR H2 in some rabbit chains. Besides, pairs of cysteine residues are often found in the CDR-L3. Moreover, many rabbit antibody CDRs do not belong to any previously known canonical structure. In particular the CDR-L3 is often much longer than the CDR-L3 of a human or murine counterpart.
Hence, the grafting of non-human CDRs antibodies into a human framework is a major protein engineering task. The transfer of antigen binding loops from a naturally evolved framework to a different artificially selected human framework must be performed so that native loop conformations are retained for antigen binding. Often antigen binding affinity is greatly reduced or abolished after loop grafting. The use of carefully selected human frameworks in grafting the antigen binding loops maximizes the probability of retaining binding affinity in the humanized molecule (Roguzka et al 1996). Although the many grafting experiments available in the literature provide a rough guide for CDR grafting, it is not possible to generalize a pattern. Typical problems consist in loosing the specificity, stability or producibility after grafting the CDR loops.
Accordingly, there is an urgent need for improved methods for reliably and rapidly humanizing rabbit antibodies for use as therapeutic and diagnostic agents. Furthermore, there is a need for human acceptor frameworks for reliably humanizing rabbit antibodies, providing functional antibodies and/or antibody fragments with drug-like biophysical properties.